What is a fold-back type drooping characteristics?

Question:

Fold-back type drooping characteristic

Answer:

This characteristic is the result of the operation of a protection circuit used so as not to exceed the safe operation area due to input overvoltage in the case of a linear-type power supply. The term "fold-back" comes from the fact that the [output voltage vs. output current] characteristic has a fold-back shape.

This type of characteristic is the result of routing the current through the protection circuit in order to keep the output current to a safe level, when the voltage between input and output (voltage between the collector and emitter of an output-stage power transistor) becomes too large and exceeds the safe operation area of the output-stage power transistor.